houses, is by no means requisite, and is often
the source of very serious evils. Were it for no other reason, than that
of avoiding the chilling influence of cold air on the tender tissue of
plants growing in a high temperature, I should feel inclined to support
such a view; but when there are facts sufficiently abundant, to prove,
that plants do not themselves vitiate the air of such structures to an
extent sufficient to render it unfit for their continued growth, or at
least, that a sufficient interchange is constantly going on, without
opening the sashes of a forcing house, the evidence appears to be
overwhelming; and the necessity of continuing a practice so fraught with
danger, and so frequently attended with disappointment, appears to be done
away.

The injury done to the tender foliage of plants in forcing houses, by
contact with cold air, results from the increased capacity of air for
moisture, as it become heated. When cold air is admitted to these
structures, it cannot contain so great a quantity of aqueous matter, as it
is capable of taking up when it becomes warmed: this increase of
temperature, is soon in great measure, supplied to it, but rarely is a
sufficient quantity of moisture, at the same time within its reach, to
enable it to supply its increased capacity for aqueous matter: the
consequence is, that on coming in contact with the foilage of the plants,
which is of a succulent nature, and contains a great proportion of water,
the warmed air continues to abstract a portion of moisture from the
plants, until its capacity is satisfied; and hence the plants are robbed
of their "life's blood." Besides this action, which is the cause of
serious evil, the tissue itself is contracted and thereby injured, by
reason of the degree of cold, which is at the first gush, liable to come
in contact with the warm foliage. These remarks apply to cold air, when
admitted in a large bulk, by opening the sashes; and when a draught is
produced, by opening them, both at the back and front, and the top and
bottom of the house.

Deterioration of the air, by the action of the functions of the plants,
could not take place, except in hermetically sealed structures: for by
reason of the expansibility and elasticity of air, when it becomes at all
heated, it not only gains egress, but also admission through the most
minute crevices: that this interchange is sufficient to counteract any
deteriorating influence which the plants might